The present invention relates to a free-running circuit arrangement for operating a load, having at least one switching element, a freewheeling diode that is connected in an anti-parallel fashion relative to the main current direction of the at least one switching element, a load circuit and a control resonant circuit which comprises at least one control inductor and at least one self-capacitance of the at least one switching element.
Such a generic free-running circuit arrangement is disclosed in DE 195 48 506. The principle in accordance with which this circuit operates can be gathered from FIG. 1 there: an inductor L2, in this case the lamp inductor, arranged in the load circuit is coupled to two auxiliary windings HW1 and HW2, the auxiliary winding HW1 being arranged in the control resonant circuit of a first switching element T1, and the auxiliary winding HW2 being arranged in the control resonant circuit of a second switching element T2. The oscillation of the circuit arrangement is maintained by electromagnetic feedback of energy from the load circuit into the respective control resonant circuit.
The disadvantage of this circuit arrangement consists in that wound items are very expensive and therefore substantially predetermine the costs of a circuit arrangement in addition to transistors.
It is therefore the object of the present invention to develop the generic free-running circuit arrangement in such a way that it is possible to maintain an oscillation without this requiring electromagnetic coupling of components of the load circuit to components of the control resonant circuit.
This object is achieved according to the invention by virtue of the fact that in the case of the generic free-running circuit arrangement the at least one switching element has a control electrode, a working electrode and a reference electrode, there acting between the control electrode and the working electrode a capacitance that is coupled to the control resonant circuit in such a way that energy is fed into the control resonant circuit by the charging and discharging current of this capacitance, the circuit arrangement having no components for feeding energy into the control resonant circuit by electromagnetic coupling.
The invention is based on the finding that the energy required to maintain the oscillation can be fed to the control resonant circuit via a capacitance. An expensive, frequently individually fabricated transformer is therefore no longer necessary.
A further advantage of the circuit arrangement according to the invention consists in that the circuit arrangement can be implemented with a smaller number of components.
In a particularly advantageous embodiment, one switching element has a Miller capacitance, the capacitance acting between the control electrode and the working electrode on the switching element comprising the Miller capacitance of the at least one switching element. Miller capacitances occur, for example, in field effect transistors. Depending on the design of the circuit, it can be possible that the Miller capacitance of the at least one switching element is alone sufficient to supply energy to the control resonant circuit for maintaining the oscillation. Should the Miller capacitance not suffice alone, an additional discrete capacitance can be connected in parallel with it.
It is therefore particularly advantageous to tune the control resonant circuit and the Miller capacitance of the at least one switching element to one another in such a way that the oscillation of the control resonant circuit is maintained solely by the charging and discharging currents of the Miller capacitance of the at least one switching element. This results in a further saving on components. Howeverxe2x80x94as mentionedxe2x80x94it can also be provided to connect an additional capacitance in parallel with the Miller capacitance of the at least one switching element, the oscillation of the control resonant circuit being maintained by the charging and discharging currents of the Miller capacitance and of the additional capacitance of the at least one switching element.
The input capacitance present between control and reference electrodes of the at least one switching element can be used as the self-capacitance of the latter.
It is preferred for the circuit arrangement according to the invention to have a first and a second switching element, the first and the second switching elements being of complementary design and being coupled to a common control resonant circuit. The complementary design of the switching elements permits the use of a common control resonant circuit for both switching elements. It is preferred in this case for each switching element to have a control electrode, a working electrode and a reference electrode, the control electrodes being connected to one another with the formation of a first tie point, and the reference electrodes being connected to one another with the formation of a second tie point, the control inductor being coupled between the first and the second tie point.
However, it can also be provided that the circuit arrangement has a first and a second switching element that are of the same type, the first switching element being coupled to a first control resonant circuit, and the second switching element being coupled to a second control resonant circuit. Again, it is preferred for each switching element to have a control electrode, a working electrode and a reference electrode, the reference electrode of the first switching element being connected to the working electrode of the second switching element, and the respective control inductor being coupled between the respective control and reference electrodes of the respective switching element. This implementation has the advantage that two switching elements of identical type can be used, the electric performance thereby also being identical.
The first and the second switching element are preferably arranged in a half-bridge arrangement.
The load is preferably an illuminating means, preferably a low-pressure discharge lamp. However, it is also possible to use the circuit for other types of loads.
For the case in which the input capacitance of the at least one switching element is unfavorably dimensioned for implementing the circuit arrangement, it is preferred to connect a discrete supplementary capacitance in parallel with the input capacitance of the at least one switching element. This option yields further degrees of freedom for dimensioning the control resonant circuit.
The load circuit preferably has a series tuned circuit with an inductor, a capacitance connected in parallel with the load and at least one decoupling capacitance. The inductor is preferably dimensioned as a current-limiting and resonance inductor.
The at least one switching element is preferably a bipolar transistor or an MOS field effect transistor. For the case in which the circuit arrangement is implemented with the aid of at least one MOS field effect transistor, the body diode of the MOS field effect transistor can implement the freewheeling diode connected in an anti-parallel fashion. A discrete diode is to be provided as freewheeling diode in the case of an implementation of the at least one switching element as bipolar transistor.
Further advantageous developments of the invention are defined in the subclaims.